Seriously, I cannot work out listening to music. I just can't concentrate on my breathing, how many reps I've done, whether or not I'm using perfect form, etc.
Once, I listened to some new Jack Johnson while I was lifting...baaaaaaaaadddddddd idea. I was too busy enjoying the music that I forgot to breath correctly. Next thing you know, I'm blacking out, hearing things that aren't there, and hunched over a trash can.

When I'm doing cardio, however, I like some fast paced rap, hip - hop, or any alternative. T.I. is one of my favorites right now, always has been, but now more than usual. Rage Against The Machine has some pretty heavy tracks that bump up your intensity.